Electrochemical Reduction of Chromate in the Presence of Nickel Chloride in Molten Lithium Chloride-Potassium Chloride Eutectic

Chronopotent iometry of chromate in the presence of NiC12 in molten LiCIKCI eutectic reveals a diffusion controlled, three electron reduct ion step. In the presence of excess NiC12, chromate is reduced at --0.35V vs. P t ( I I ) / P t reference electrode and the electroactive species responsible for the chronopotentiometric wave is estimated to have a diffusion coefficient of 1.06 9 10 -5 cm 2 sec I at 450~ The stoichiometry of the reduct ion product depends main ly on the tempera ture at which the deposit is formed. At 500~ the deposit approaches the composition LiNi2CrO4. X r a y powder diffraction shows the deposit to be a single compound with a face centered cubic lattice. The length of the uni t cell edge is estimated to be 4.14A. At 1400~ a weight loss of the deposit is observed. Xray powder diffraction shows the presence of two phases, which are identified as NiO and NiCr204. The weight loss is a t t r ibuted to volatization of Li20. Lai t inen and Propp (1) have shown that the elect rcchemical reduction product of K2CrO4 in LiC1-KC1 eutectic containing dissolved MgC12 is a single unstoichiometric compound of formula LixMgyCrO4, where x W 2y -~ 5. The values of x and y depended on the conditions of the electrolysis, namely current density, temperature , and the molar ratio of Mg (II) to Cr(VI) dissolved in the melt. Typical values of x ranged between 0.3 and 0.5. Hanck (2) observed that the reduction of chromate in the presence of Zn( I I ) was shifted from --1.0V vs. P t ( I I ) / P t reference to --0.5V. Analysis of the deposit indicated the composition to be LiZn~CrO4. The stoichiometry of the compound was not affected by the electrolysis conditions. The present investigation was under taken to determine whether the reduction of K2CrO4 is affected by the presence of NiC12 and to establish the composit ion of the reduct ion product. Spectrophotometric measurements failed to indicate any interact ion between Ni( I I ) and chromate in the melt (3). Ni( I I ) is known to exist as stable chlorocomplexes (4) in chloride melts, and it was of interest to determine whether any chloride is contained in the deposit. Also the reduction potent ial of Ni( I I ) lies about 0.2V below that of chromate in the absence of divalent ions and it was of interest to determine whether the deposition of metallic nickel could be avoided.